Superalloy mortar tube

ABSTRACT

A finless mortar tube made of a superalloy includes, seriatim, a breech end, a beginning taper point, an ending taper point, a lower clamp region, an upper clamp region, and a muzzle end. The nominal wall thickness of the tube is constant from forward of the breech end to the beginning taper point and the nominal wall thickness of the tube decreases from the beginning taper point to the ending taper point. The mortar tube is capable of a substantial increase in the rate of fire compared to conventional mortar tubes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/162,745 filed on Sep. 21, 2005, which claims the benefitunder 35 USC 119(e) of U.S. provisional patent applications 60/522,510filed on Oct. 7, 2004 and 60/522,566 filed on Oct. 14, 2004, whichapplications are hereby incorporated by reference.

STATEMENT OF GOVERNMENT INTEREST

The inventions described herein may be manufactured, used and licensedby or for the U.S. Government for U.S. Government purposes.

BACKGROUND OF THE INVENTION

The invention relates in general to mortar tubes, and in particular tofinless mortar tubes with reduced wall thicknesses.

Mortars tubes presently used by the United States armed forces aregenerally available in three sizes of nominal inside diameter, namely,60 mm (millimeter), 81 mm and 120 mm. The current 60 mm and 81 mm mortartubes have cooling fins that function to reduce the tube temperatureduring firing. The mortar tube cooling fins are expensive to manufactureand add additional weight to the mortar tube. The 120 mm mortar tubedoes not have cooling fins because its required rate of fire is lessthan the 60 mm and 81 mm mortars. Lightweight finless mortar tubes inthe 60 mm and 81 mm sizes that are capable of firing high pressurerounds at the high rates of fire characteristic of United States mortarsare not known.

Generally speaking, the soldier in the field benefits whenever anythinghe/she must handle is made to weigh less. In “Hydrostatic Extrusion of60 mm Mortar Tubes” (Watervliet Arsenal, Watervliet, N.Y., October 1974,available from NTIS, Springfield, Va.), DeFries describes thehydrostatic extrusion of four 60 mm tubes made of Inconel, a“superalloy.” These tubes were relatively thick-walled (approximately 5mm or greater) and included cooling fins. Although some mechanical testswere performed on the DeFries tubes, it does not appear that the tubeswere ever “live-fire” tested. There is a need for a mortar tube that islight in weight (thin-walled), cheap to manufacture (no cooling fins),and capable of rapid, continuous firing without failure.

SUMMARY OF THE INVENTION

An object of the invention is to provide mortar tubes that are lighterin weight than known mortar tubes.

Another object of the invention is to provide finless mortar tubes inthe 60 mm and 81 mm sizes.

A further object of the invention is to provide light-weight, finlessmortar tubes that can withstand rapid, continuous firing rates withoutplastic deformation.

One aspect of the invention is a mortar tube comprising a tube having nocooling fins, made of a superalloy, and having a nominal constant insidediameter of about 60 mm; the tube comprising, seriatim, a breech end, abeginning taper point, an ending taper point, a lower clamp region, anupper clamp region, and a muzzle end; a nominal wall thickness of thetube being constant from forward of the breech end to the beginningtaper point and the nominal wall thickness of the tube decreasing fromthe beginning taper point to the ending taper point; wherein the tubedoes not undergo plastic deformation when firing 30 rounds per minutefor four minutes and 20 rounds per minute continuous thereafter, at amaximum pressure of about 10,080 psi.

Another aspect of the invention is a mortar tube comprising a tubehaving no cooling fins, made of a superalloy, and having a nominalconstant inside diameter of about 81 mm; the tube comprising, seriatim,a breech end, a beginning taper point, an ending taper point, a lowerclamp region, an upper clamp region, and a muzzle end; a nominal wallthickness of the tube being constant from forward of the breech end tothe beginning taper point and the nominal wall thickness of the tubedecreasing from the beginning taper point to the ending taper point;wherein the tube does not undergo plastic deformation when firing 30rounds per minute for two minutes and 15 rounds per minute continuousthereafter, at a maximum pressure of about 15,800 psi.

Further aspects of the invention are methods of making mortar tubes fromsuperalloys.

The invention will be better understood, and further objects, features,and advantages thereof will become more apparent from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily to scale, like orcorresponding parts are denoted by like or corresponding referencenumerals.

FIG. 1A is a side view of a known mortar tube.

FIG. 1B is a sectional view taken along the line 1B-1B of FIG. 1A.

FIG. 2 is a graph of tube temperature vs. axial position for two finlesstubes.

FIG. 3 is a side view, partially in section, of one embodiment of a 60mm tube in accordance with the invention.

FIG. 4 is a side view, partially in section, of one embodiment of an 81mm tube in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention are mortar tubes that do nothave the cooling fins of conventional mortar tubes. The inventive mortartubes are made of a high strength superalloy. Superalloys are known andtypically fall into one of three types, iron based, cobalt based andnickel based. In general, the superalloys have material strengthsgreater than 140 ksi at tube temperatures greater than 1000 degreesFahrenheit. The use of a higher strength material permits a thinner wallthickness, as compared to conventional tubes. The mortar tubes madeaccording to the invention weigh approximately thirty percent less thanconventional mortar tubes.

Many complex, interrelated and often conflicting factors influence thewall thickness of a mortar tube. Through years of work, the inventorshave developed tube profiles for 60 mm and 81 mm finless mortar tubesmade of a superalloy, such as Inconel 718 Factors considered include,inter alia, interior ballistics, heat transfer, temperature and pressureprofiles, amount of charge per round (charge one to charge four),projectile weight, required rate of fire, wearing of the tube thickness,and stresses not induced by firing, for example, impact stresses causedby dropping the tube on the ground and stresses caused by attachingother components to the tube, such as a bipod. Both computer simulationsand live firing test methods were used. Another factor that influencesthe wall thickness is the manufacturing method. Tubes of a giventhickness profile may be satisfactorily produced using one manufacturingmethod, but another manufacturing method may require adjustments to thetube thickness.

FIG. 1A is a side view of a known 81 mm mortar tube 10 and FIG. 1B is asectional view of the tube 10 taken along the line 1B-1B of FIG. 1A.Tube 10 includes cooling fins 12 on the rear portion near the breech. Aseparate blast attenuation device (BAD) 14 is attached at the muzzle endof the tube 10. As seen in FIG. 1B, tube 10 has a wall thickness g. Thecooling fins 12 reduce the temperature of the mortar tube 10 from about1160° F. to 1022° F. at presently required maximum rates of fire, i.e.,30 rounds per minute for 2 minutes and 15 rounds per minute sustained.These rates of fire are based on mortar ammunition having maximum designpressures of 15,800 psi. The steel used to make tube 10 cannot withstandthe design ammo pressure loads if the tube temperature increases above1160° F., as it would if the tube 10 had no cooling fins 12.

The rate of fire (ROF) in number of rounds per minute (rds/min) is animportant factor in determining the temperatures that a mortar tube willexperience. The higher the ROF number, the higher the temperatures themortar tube will experience. For an 81 mm finless mortar tube ofconventional construction, the maximum ROF is 25 rds/min for 1 minuteand 5 rds/min sustained. The conventional tube has a low ROF and isunable to satisfy future requirements for operational use.

In the invention, the performance criteria for the 60 mm and 81 mm tubesinvolve worst case firing conditions wherein the ambient air temperatureis 145 F (63 C) and there is no wind related cooling (a calm day). Inaccordance with the requirements of STANAG 4110 (A NATO STANardizationAGreement for the Definition of Pressure Terms and TheirInterrelationship for Use in the Design and Proof of Cannons or Mortarsand Ammunition), the mortar tube must be able to function within itsdesign requirements without undergoing plastic deformation when firing a1 in a million max pressure.

The ROF for the inventive 60 mm tube is 30 rounds per minute for 4minutes and 20 rounds per minute continuous thereafter. The 60 mm roundis a charge four round with a 3 lbm projectile. The one in a millionpressure for the 60 mm tube is 10,080 psi. For the 60 mm tube, thehottest temperature occurs during the first 4 minutes when the barrel isbeing fired at 30 rpm.

The ROF for the inventive 81 mm tube is 30 rounds per minute for 2minutes and 15 rounds per minute continuous thereafter. The 81 mm roundis a charge four round with a 9 lbm projectile. The one in a millionpressure for the 81 mm tube is 15,800 psi. For the 81 mm tube, thehottest temperature is reached during the sustained firing period at 15rpm.

FIG. 2 graphically shows temperature profile vs. axial position in thetube for a conventional 81 mm tube (lower curve) and the inventive 81 mmtube (upper curve) at each tube's maximum permissible ROF. The inventivetube's temperature is approximately 400° F. hotter, because of theability to handle a larger ROF. The conventional mortar tube cannothandle an increased ROF, as needed to meet future requirements, withoutadding cooling fins.

FIG. 3 is a side view, partially in section, of one embodiment of a 60mm mortar tube 20 in accordance with the invention. Tube 20 has nocooling fins and is made of a superalloy. The superalloy may be one ofnickel based, iron based or cobalt based. An example of a nickel basedsuperalloy is Inconel.

Tube 20 has a nominal constant inside diameter of about 60 mm. Tube 20includes a breech end 22, a beginning taper point 24, an ending taperpoint 26, a lower clamp region 28, an upper clamp region 30, and amuzzle end 32. In the embodiment of FIG. 3, a separate base cap (notshown) is attached to breech end 22. However, the breech end may also bemanufactured with an integral base cap. Tube 20 does not undergo plasticdeformation when firing 30 rounds per minute for four minutes and 20rounds per minute continuous thereafter, the rounds being charge fourrounds with projectiles of three lbm. Tube 20 will perform as stated forat least 10,000 rounds.

The nominal wall thickness a of the tube 20 is constant from forward ofthe breech end 22 to the beginning taper point 24. The nominal constantwall thickness a is in a range of about 2.6 mm to about 3 mm and morepreferably in a range of about 2.6 mm to about 2.83 mm.

The nominal wall thickness b of the tube 20 decreases from the beginningtaper point 24 to the ending taper point 26. The taper of the outsidesurface of the tube 20 from the beginning taper point 24 to the endingtaper point 26 is in a range of about −0.44 degrees to about −0.55degrees and more preferably in a range of about −0.47 degrees to about−0.51 degrees. In one embodiment, the nominal wall thickness at thebeginning taper point is about 2.6 mm and the nominal wall thickness atthe ending taper point is about 1.67 mm. In another embodiment, thenominal wall thickness at the beginning taper point is about 2.83 mm andthe nominal wall thickness at the ending taper point is about 1.97 mm.

The distance e from the muzzle end 32 to the beginning taper point 24 isin a range of about 590 mm to about 600 mm. The distance f from themuzzle end 32 to the ending taper point 26 is in a range of about 485 mmto about 495 mm. In a preferred embodiment, the distance e is about 594mm and the distance f is about 490 mm.

The nominal wall thickness c may be constant from the ending taper point26 to aft of the lower clamp region 28. The nominal constant wallthickness c from the ending taper point 26 to aft of the lower clampregion 28 may be in a range of about 1.67 mm to about 1.97 mm.

The nominal wall thickness d from the lower clamp region 28 to the upperclamp region 30 and from the upper clamp region 30 to the muzzle end 32may be constant. The nominal constant wall thickness d may be in a rangeof about 1.5 mm to about 2 mm. In one embodiment, the constant nominalwall thickness d is about 1.55 mm.

The edges 34, 36, 38, 40 of the lower and upper clamp regions 28, 30have wall thicknesses that are greater than the adjacent nominal wallthicknesses. The wall thickness of the central area 42 of lower clampregion 28 may be about 1.85 mm. The wall thickness of the central area44 of the upper clamp region 30 may be about 1.55 mm. Other areas ofincreased wall thickness include the muzzle end 32, the breech end 22and rings 46, 48. Rings 46, 48 may be used to locate and contain a steelband (not shown) used to arm a projectile.

Tube 20 may be formed by, for example, forging and machining, or a metalflow-forming process. In general, if the tube 20 is forged and machined,the thinner wall thicknesses may be used. If the tube 20 is flow-formed,then the thicker wall thicknesses may be used.

FIG. 4 is a side view, partially in section, of one embodiment of an 81mm mortar tube 50 in accordance with the invention. Tube 50 has nocooling fins and is made of a superalloy. The superalloy may be one ofnickel based, iron based or cobalt based. An example of a nickel basedsuperalloy is Inconel.

Tube 50 has a nominal constant inside diameter of about 81 mm. Tube 50includes a breech end 52, a beginning taper point 54, ending taperpoints 56 and 56′, a lower clamp region 58, an upper clamp region 60, ablast attenuation device 61, and a muzzle end 62. In the embodiment ofFIG. 4, a separate base cap (not shown) is attached to breech end 52.However, the breech end may also be manufactured with an integral basecap. Tube 50 does not undergo plastic deformation when firing 30 roundsper minute for two minutes and 15 rounds per minute continuousthereafter, the rounds being charge four rounds with projectiles of ninelbm. Tube 50 will perform as stated for at least 10,000 rounds.

The nominal wall thickness h of the tube 50 is constant from forward ofthe breech end 52 to the beginning taper point 54. The nominal constantwall thickness h is in a range of about 4.97 mm to about 5.7 mm and morepreferably in a range of about 4.97 mm to about 5.42 mm.

The nominal wall thickness i of the tube 50 decreases from the beginningtaper point 54 to the ending taper point 56 or 56′. The taper of theoutside surface of the tube 50 from the beginning taper point 54 to theending taper point 56 or 56′ is in a range of about −0.60 degrees toabout −0.90 degrees and preferably in a range of about −0.70 degrees toabout −0.83 degrees.

In one embodiment, the nominal wall thickness at the beginning taperpoint 54 is about 4.97 mm and the nominal wall thickness at the endingtaper point 56′ is about 2.8 mm. In another embodiment, the nominal wallthickness at the beginning taper point 54 is about 5.42 mm and thenominal wall thickness at the ending taper point 56 is about 3.26 mm.

The distance k from the muzzle end 62 to the beginning taper point 54 isin a range of about 880 mm to about 890 mm from the muzzle end 62.Preferably, the distance k is about 886 mm. In one embodiment, theending taper point 56′ is just aft of the lower clamp region 58. Inanother embodiment, the distance l from the muzzle end 62 to the endingtaper point 56 is in the range of about 730 mm to about 740 mm and thenominal constant wall thickness j from the ending taper point 56 to thelower clamp region 58 is in a range of about 3.24 mm to about 3.28 mm.Preferably, the distance l is about 737 mm.

The nominal wall thickness m decreases from the lower clamp region 58 tothe upper clamp region 60. The taper of the outside surface of the tube50 from the lower clamp region 58 to the upper clamp region 60 may be ina range of about −0.13 degrees to about 0.17 degrees. In one embodiment,the nominal wall thickness m decreases from about 2.1 mm forward of thelower clamp region 58 to about 1.61 mm aft of the upper clamp region 60.In another embodiment, the nominal wall thickness m decreases from about2.21 mm forward of the lower clamp region 58 to about 1.71 mm aft of theupper clamp region 60.

A constant nominal wall thickness n from the upper clamp region 60 toaft of the blast attenuation device 61 may be in a range of about 1.6 mmto about 1.9 mm. Preferably, the constant nominal wall thickness n fromthe upper clamp region 60 to aft of the blast attenuation device 61 isabout 1.63 mm.

The edges 64, 66, 68, 70 of the lower and upper clamp regions 58, 60have wall thicknesses that are greater than the adjacent nominal wallthicknesses. The wall thickness of the central area 72 of lower clampregion 58 may be about 2.55 mm. The wall thickness of the central area74 of the upper clamp region 60 may be about 2.55 mm. Other areas ofincreased wall thickness include the muzzle end 62 and the breech end52.

Tube 50 may be formed by, for example, forging and machining, or a metalflow-forming process. In general, if the tube 50 is forged and machined,the thinner wall thicknesses may be used. If the tube 50 is flow-formed,then the thicker wall thicknesses may be used. Whether forged andmachined, flow-formed or made with some other technique, the blastattenuation device 61 is formed integrally with the tube 50. In thepast, the device 61 was a separate component that had to be added to thetube 50 after manufacture. Adding the device 61 to the tube 50 aftermanufacture was a costly process.

While the invention has been described with reference to certainpreferred embodiments, numerous changes, alterations and modificationsto the described embodiments are possible without departing from thespirit and scope of the invention as defined in the appended claims, andequivalents thereof.

1. A mortar tube, comprising: a tube having no cooling fins, made of asuperalloy, and having a nominal constant inside diameter of about 60mm; the tube comprising, seriatim, a breech end, a beginning taperpoint, an ending taper point, a lower clamp region, an upper clampregion, and a muzzle end; a nominal wall thickness of the tube beingconstant from forward of the breech end to the beginning taper point andthe nominal wall thickness of the tube decreasing from the beginningtaper point to the ending taper point; wherein the tube does not undergoplastic deformation when firing 30 rounds per minute for four minutesand 20 rounds per minute continuous thereafter, at a maximum pressure ofabout 10,080 psi.
 2. The mortar tube of claim 1 wherein the nominalconstant wall thickness from forward of the breech end to the beginningtaper point is in a range of about 2.6 mm to about 3 mm.
 3. The mortartube of claim 2 wherein the nominal constant wall thickness from forwardof the breech end to the beginning taper point is in a range of about2.6 mm to about 2.83 mm.
 4. The mortar tube of claim 1 wherein a taperof an outside surface of the tube from the beginning taper point to theending taper point is in a range of about −0.44 degrees to about −0.55degrees.
 5. The mortar tube of claim 4 wherein the taper of the outsidesurface of the tube from the beginning taper point to the ending taperpoint is in a range of about −0.47 degrees to about −0.51 degrees. 6.The mortar tube of claim 1 wherein the nominal wall thickness isconstant from the ending taper point to aft of the lower clamp region.7. The mortar tube of claim 6 wherein the nominal constant wallthickness from the ending taper point to aft of the lower clamp regionis in a range of about 1.67 mm to about 1.97 mm.
 8. The mortar tube ofclaim 1 wherein the nominal wall thickness at the beginning taper pointis about 2.6 mm and the nominal wall thickness at the ending taper pointis about 1.67 mm.
 9. The mortar tube of claim 1 wherein the nominal wallthickness at the beginning taper point is about 2.83 mm and the nominalwall thickness at the ending taper point is about 1.97 mm.
 10. Themortar tube of claim 1 wherein the nominal wall thickness from the lowerclamp region to the upper clamp region and from the upper clamp regionto the muzzle end is constant.
 11. The mortar tube of claim 10 whereinthe constant nominal wall thickness from the lower clamp region to theupper clamp region and from the upper clamp region to the muzzle end isin a range of about 1.5 mm to about 2 mm.
 12. The mortar tube of claim11 wherein the constant nominal wall thickness from the lower clampregion to the upper clamp region and from the upper clamp region to themuzzle end is about 1.55 mm.
 13. The mortar tube of claim 1 wherein thebeginning taper point is in a range of about 590 mm to about 600 mm fromthe muzzle end of the tube and the ending taper point is in a range ofabout 485 to about 495 mm from the muzzle end.
 14. The mortar tube ofclaim 13 wherein the beginning taper point is about 594 mm from themuzzle end of the tube and the ending taper point is about 490 mm fromthe muzzle end.
 15. A mortar tube, comprising: a tube having no coolingfins, made of a superalloy, and having a nominal constant insidediameter of about 81 mm; the tube comprising, seriatim, a breech end, abeginning taper point, an ending taper point, a lower clamp region, anupper clamp region, and a muzzle end; a nominal wall thickness of thetube being constant from forward of the breech end to the beginningtaper point and the nominal wall thickness of the tube decreasing fromthe beginning taper point to the ending taper point; wherein the tubedoes not undergo plastic deformation when firing 30 rounds per minutefor two minutes and 15 rounds per minute continuous thereafter, at amaximum pressure of about 15,800 psi.
 16. The mortar tube of claim 15wherein the nominal constant wall thickness from forward of the breechend to the beginning taper point is in a range of about 4.97 mm to about5.7 mm.
 17. The mortar tube of claim 16 wherein the nominal constantwall thickness from forward of the breech end to the beginning taperpoint is in a range of about 4.97 mm to about 5.42 mm.
 18. The mortartube of claim 15 wherein a taper of an outside surface of the tube fromthe beginning taper point to the ending taper point is in a range ofabout −0.60 degrees to about −0.90 degrees.
 19. The mortar tube of claim18 wherein the taper of the outside surface of the tube from thebeginning taper point to the ending taper point is in a range of about−0.70 degrees to about −0.83 degrees.
 20. The mortar tube of claim 15wherein the nominal wall thickness at the beginning taper point is about4.97 mm and the nominal wall thickness at the ending taper point isabout 2.8 mm.
 21. The mortar tube of claim 15 wherein the nominal wallthickness at the beginning taper point is about 5.42 mm and the nominalwall thickness at the ending taper point is about 3.26 mm.
 22. Themortar tube of claim 15 wherein the beginning taper point is a range ofabout 880 mm to about 890 mm from the muzzle end.
 23. The mortar tube ofclaim 15 wherein the ending taper point is at an aft end of the lowerclamp region.
 24. The mortar tube of claim 15 wherein the ending taperpoint is in the range of about 730 mm to about 740 mm from the muzzleend and the nominal constant wall thickness from the ending taper pointto the lower clamp region is in a range of about 3.24 mm to about 3.28mm.
 25. The mortar tube of claim 15 wherein the nominal wall thicknessdecreases from the lower clamp region to the upper clamp region.
 26. Themortar tube of claim 25 wherein a taper of an outside surface of thetube from the lower clamp region to the upper clamp region is in a rangeof about −0.13 degrees to about −0.17 degrees.
 27. The mortar tube ofclaim 25 wherein the nominal wall thickness decreases from about 2.11 mmforward of the lower clamp region to about 1.61 mm aft of the upperclamp region.
 28. The mortar tube of claim 25 wherein the nominal wallthickness decreases from about 2.21 mm forward of the lower clamp regionto about 1.71 mm aft of the upper clamp region.
 29. The mortar tube ofclaim 15 further comprising a blast attenuation device at the muzzle endand wherein a constant nominal wall thickness from the upper clampregion to aft of the blast attenuation device is a range of about 1.6 mmto about 1.9 mm.
 30. The mortar tube of claim 29 wherein the constantnominal wall thickness from the upper clamp region to aft of the blastattenuation device is about 1.63 mm.
 31. A method of making a mortartube, comprising: providing a superalloy material; and making the mortartube of claim 1 from the superalloy material.
 32. A method of making amortar tube, comprising: providing a superalloy material; and making themortar tube of claim 15 from the superalloy material.
 33. The method ofclaim 32 wherein the making step includes forming a blast attenuationdevice integral with the mortar tube.